Re: [PATCH v3 1/1] psi: stop relying on timer_pending for poll_work rescheduling

[Suren Baghdasaryan <surenb@google.com>]

On Thu, Jul 8, 2021 at 11:38 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> On Thu, Jul 08, 2021 at 08:54:56AM -0700, Suren Baghdasaryan wrote:
> > On Thu, Jul 8, 2021 at 7:44 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
> > > On Wed, Jul 07, 2021 at 03:43:48PM -0700, Suren Baghdasaryan wrote:
> > > > On Wed, Jul 7, 2021 at 6:39 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
> > > > > This looks good to me now code wise. Just a comment on the comments:
> > > > >
> > > > > On Tue, Jul 06, 2021 at 07:39:33PM -0700, Suren Baghdasaryan wrote:
> > > > > > @@ -559,18 +560,14 @@ static u64 update_triggers(struct psi_group *group, u64 now)
> > > > > >       return now + group->poll_min_period;
> > > > > >  }
> > > > > >
> > > > > > -/* Schedule polling if it's not already scheduled. */
> > > > > > -static void psi_schedule_poll_work(struct psi_group *group, unsigned long delay)
> > > > > > +/* Schedule polling if it's not already scheduled or forced. */
> > > > > > +static void psi_schedule_poll_work(struct psi_group *group, unsigned long delay,
> > > > > > +                                bool force)
> > > > > >  {
> > > > > >       struct task_struct *task;
> > > > > >
> > > > > > -     /*
> > > > > > -      * Do not reschedule if already scheduled.
> > > > > > -      * Possible race with a timer scheduled after this check but before
> > > > > > -      * mod_timer below can be tolerated because group->polling_next_update
> > > > > > -      * will keep updates on schedule.
> > > > > > -      */
> > > > > > -     if (timer_pending(&group->poll_timer))
> > > > > > +     /* xchg should be called even when !force to set poll_scheduled */
> > > > > > +     if (atomic_xchg(&group->poll_scheduled, 1) && !force)
> > > > > >               return;
> > > > >
> > > > > This explains what the code does, but not why. It would be good to
> > > > > explain the ordering with poll_work, here or there. But both sides
> > > > > should mention each other.
> > > >
> > > > How about this:
> > > >
> > > > /*
> > > >  * atomic_xchg should be called even when !force to always set poll_scheduled
> > > >  * and to provide a memory barrier (see the comment inside psi_poll_work).
> > > >  */
> > >
> > > The memory barrier part makes sense, but the first part says what the
> > > code does and the message is unclear to me. Are you worried somebody
> > > might turn this around in the future and only conditionalize on
> > > poll_scheduled when !force? Essentially, I don't see the downside of
> > > dropping that. But maybe I'm missing something.
> >
> > Actually you are right. Originally I was worried that there might be a
> > case when poll_scheduled==0 and force==true and if someone flips the
> > conditions we will reschedule the timer but will not set
> > poll_scheduled back to 1.
>
> Oh I see.
>
> Right, flipping the condition doesn't make sense because we need
> poll_scheduled to be set when we go ahead - whether we're forcing or
> not. I.e. if we were in a locked section, we'd write it like this:
>
>         if (poll_scheduled)
>                 if (!force)
>                         return;
>         else
>                 poll_scheduled = 1;
>
> > However I don't think this condition is possible. We set force=true
> > only when we skipped resetting poll_schedule to 0 and on initial
> > wakeup we always reset poll_schedule. How about changing the comment
> > to this:
> >
> >  /*
> >   * atomic_xchg should be called even when !force to provide a
> >   * full memory barrier (see the comment inside psi_poll_work).
> >   */
>
> Personally, I still find this more confusing than no comment on
> !force, because when you read it it sort of raises the question what
> the alternatives would be. And the alternatives appear to be
> nonsensical code rather than legitimate options.
>
> But I won't insist if you prefer to leave it in. Your call.

I would like to keep it as a precaution, if you don't mind. In case
someone in the future thinks about "optimizing" this by flipping the
condition, hopefully the comment will give them a pause to think about
it :)

>
> > > /*
> > >  * A task change can race with the poll worker that is supposed to
> > >  * report on it. To avoid missing events, ensure ordering between
> > >  * poll_scheduled and the task state accesses, such that if the poll
> > >  * worker misses the state update, the task change is guaranteed to
> > >  * reschedule the poll worker:
> > >  *
> > >  * poll worker:
> > >  *   atomic_set(poll_scheduled, 0)
> > >  *   smp_mb()
> > >  *   LOAD states
> > >  *
> > >  * task change:
> > >  *   STORE states
> > >  *   if atomic_xchg(poll_scheduled, 1) == 0:
> > >  *     schedule poll worker
> > >  *
> > >  * The atomic_xchg() implies a full barrier.
> > >  */
> > >  smp_mb();
> > >
> > > This gives a high-level view of what's happening but it can still be
> > > mapped to the code by following the poll_scheduled variable.
> >
> > This looks really good to me.
> > If you agree on the first comment modification, should I respin the
> > next version?
>
> Yeah, sounds good to me!

Thanks! I'll post an update shortly.